1. Field of the Invention
The present invention relates to a fuel-injection system in which fuel is charged into combustion chambers in accordance with the hydraulic pressure of a hydraulically actuated fluid supplied from an accumulator.
2. Description of the Prior Art
An electronically-controlled fuel-injection system has been well known in which injectors are each provided with a needle valve movable in an injector body in a reciprocating manner to open and close injection holes, and an electronic device applied with an actuating current so as to control the hydraulically actuated fluid for driving the needle valve upwards and downwards, whereby the fuel to be injected out of the injectors is regulated in injection timing and amount to be injected per cycle by a controller unit, depending on the operating conditions of the engine.
There have been conventionally proposed two types of the electronically-controlled fuel-injection system, one of which is a fuel-injection system of hydraulically actuated fuel type in which a high-pressure fuel stored in a common fuel supply rail, for example, is used in place of the hydraulically actuated fluid, and comprised of injectors each having a main body provided therein with a pressure control chamber for the high-pressure fuel, and a solenoid-operated valve for the electronic device to control an ingress and egress of the hydraulically actuated fuel into and out of the high-pressure chamber whereby the needle valve is made to move upwards and downwards in response to the hydraulic force of the high-pressure fuel in the high-pressure chamber so as to inject the pressurized fuel through the injection holes that have been free from the needle valve. Another type of the injector is a fuel-injection system of hydraulically actuated oil type in which a high-pressure engine oil is used as the hydraulically actuated fluid, and comprised of injectors each having a solenoid-operated valve for the electronic device to control an ingress the hydraulically actuated engine oil, and a boosting piston actuated with the admitted engine oil so as to intensify the fuel in an intensified chamber whereby the pressure of the pressurized fuel makes the needle valve move upwards and downwards so as to inject the pressurized fuel through the injection holes that have been free from the needle valve. In either type of the fuel-injection system, the actuation of the electronic device, or the solenoid-actuated valve, is controlled with the actuating current output from an electronically-controlling apparatus, or a controller unit. Upon lifting the needle valve by the action of the hydraulically actuated fluid that is intensified to a high pressure and supplied into the injectors with the operation of the electronic controller unit, the fuel is injected out of the injection holes at a preselected amount and at a preselected injection timing.
Most electronically-controlled fuel-injection systems as described just above usually determine the actual amount of the injected fuel with both of the pressure of the hydraulically actuated fluid and the conductive pulse width to the electronic device. The controller unit is stored with data regarding correlation of the desired amount of injected fuel versus the conductive pulse width for every pressure of the hydraulically actuated fluid and refers to the correlation data applicable to the pressure of the hydraulically actuated fluid and the desired volume of the injected fuel at some selected time, resulting in determining the conductive pulse width that corresponds to the length of time during which the electronic device is energized. FIG. 11 is a graphical representation showing a correlation data of desired volume of injected fuel versus pulse width at some selected injection pressure in a prior fuel-injection system for engines. In this correlation, the data given at specified points from previous experiments are stored in the form of a map marked with x in FIG. 11 while the conductive pulse width between the adjoining specified points, or marks x, is given by the process of interpolation. When a minute fuel injection is required in the electronically-controlled fuel-injection systems, the conductive pulse width for actuating the electronic device to this end is given by interpolating the range between the origin and the minimum value out of the data of the specified points in FIG. 11.
A version of fuel-injection system has been known in which the desired amount of fuel to be injected per one cycle is divided into a main injection part and a pilot injection part of minute amount, which is charged in advance of the main injection to thereby reduce the initial heat release rate of the combustion chamber, resulting in accomplishing the reduction in noise and NOx emissions. Disclosed in Japanese Patent Laid-Open No. 302537/1993 is an example of an electronically-controlled fuel-injection system for regulating the fuel injection by means of the solenoid-operated valve operable as described above. The fuel-injection system in the above citation is a common-rail fuel-injection system having a pilot injection executable mode. This prior system, when the pilot injection mode and the starter are simultaneously in the "on" state, may identify whether the common-rail pressure and its rise rate are less than the predetermined levels. If the pressure situations are less, the pilot injection mode is turned off to thereby terminate the pilot injection. Accordingly, there is no automatic execution of the pilot injection instruction under the event of cranking the engine where the common-rail pressure is insufficient, resulting in eliminating the repeated waste leakage of fuel. No fuel injection out of the injectors is carried out with a command pulse width less than the minimum command pulse width Pwmin. That is, the fuel injection inevitably requires the application of a command pulse having a pulse width more or equal to the width Pwmin. It is to be noted that the minimum command pulse width, shown with a broken line Pwmin in FIG. 11, during which the fuel injection may be carried out is actually different at every injector owing to the aging and scattering in characteristics inherent in each injector. In some cases, the fuel injection failure, where the amount of the injected fuel is 0, may happen whether the electronic device, or the solenoid-actuated valve, is conducting with the conductive pulse width calculated corresponding to the desired amount of the fuel injection. The minimum command pulse width at every injector is usually unknown at which the fuel injection will start. Even if the minimum command pulse width Pwmin were given initially, it would undergo a change due to the aging. The events of the fuel injection failure as described above are apt to remarkably occur in the case of the prior pilot injection. In these events, not only no expected advantage of the pilot injection is accomplished, but also a major problem arises of increasing the noise and the amount of NOx emissions. Also in the engine small in displacement per a cylinder, the pilot injection becomes the origin of a scattering or variation in the amount of the injected fuel in the idling during which the desired amount of the injected fuel reduces in the minute fraction, resulting in disturbing the steady idling of the engine, that is, large variation in the rotational frequencies.
In the meantime the pressure in the common fuel supply rail in the electrically-controlled fuel-injection system as described above drops down once the fuel is injected actually out of the injectors. This teaches the pressure change in the common fuel sully rail relates to the fuel injection out of the injectors. In the fuel-injection control, accordingly, the common rail pressure is particular one of the necessary information for determining the fuel-injection duration during which the desired amount of the fuel is injected according to the calculation. To this end, pressure sensors have been already incorporated in the common rail in the prior fuel-injection system in order to pick up the pressure in the common rail. On the technology trend as described above, a concept has happened whether it is possible to make the fuel-injection system to learn the minimum pulse width necessary for beginning the fuel injection at every injector by making use of the data concerning with the common rail pressure detected at the pressure sensor.